[M.A.D.M.A.N.]

End лишний.
И учитесь уже код нормально оформлять. читать же невозможно!

Код:

procedure TForm1.Button1Click(Sender: TObject);
  var
    Xmin, Xmax, h, y: real;
    cx, cy: real;
  begin
    try
      Xmin := StrToFloat(LabeledEdit1.Text);
      Xmax := StrToFloat(LabeledEdit2.Text);
      h := StrToFloat(LabeledEdit3.Text);
    except
      MessageDlg('неверные данные', mtError, [mbOK], 0);
      exit
    end;
    if Xmax <= Xmin then
    begin
      MessageDlg('Xmin>Xmax!', mtError, [mbOK], 0);
      exit
    end;

    Memo1.Lines.Clear;
    with Image1.Canvas do
    begin
      Brush.Color := clBlue;
      FillRect(Image1.ClientRect);
      pen.Color := clWhite;
      moveto(0, Image1.Height div 2);
      lineto(Image1.Width, Image1.Height div 2);
      cx := (Xmax - Xmin) / Image1.Width;
      cy := 2 / Image1.Height;
      if cy > cx then
        cx := cy;
      moveto(0, Image1.Height div 2);
      While Xmin <= Xmax do
      begin
        y := sin(Xmin);
        Memo1.Lines.Add('sin(' + FloatToStrF(Xmin, ffFixed, 10, 4) + ')=' +
          FloatToStrF(y, ffFixed, 10, 4));
        lineto(trunc(Xmin / cx), Image1.Height div 2 - trunc(Xmin / cx));
        Xmin := Xmin + h;
      end;
    end;
  end;